Robyn Williams: Our PhD this week is from La Trobe University in Melbourne, though he spends most of his time on Christmas Island. Luke O’Loughlin, on invasive species:
Luke O’Loughlin: Human activities are significantly transforming natural environments. One key consequence of human success has been the movement of many species beyond the limits of their native ranges into areas they do not naturally occur. Now there is essentially no ecosystem that is free of exotic species. Yet not all species become invasive and cause impact. Some are unsuccessful and never establish.
It has been increasingly recognised that the properties of the recipient community play a key role in either allowing or inhibiting the entry and spread of an invader. However, what is also evident is that successful invaders can, through their impacts, alter key properties of ecosystems that they invade, thereby creating altered communities. And it’s in these altered communities that previously unsuccessful exotic species may now have an opportunity to enter and become invasive themselves. In a time where the eradication of many invasive species is simply not practical, understanding the kinds of interactions that conform is vital to the conservation effort of these now novel ecosystems.
For my PhD research I’m thinking about these kinds of issues in the context of exotic species interactions on Christmas Island. Due to its isolation and position within the Indian Ocean, Christmas Island has evolved a unique tropical ecosystem with high species endemism. Essentially all ecosystem processes in the rainforest there are driven by the highly abundant native red land crab. This species is found in natural densities of almost one crab per square metre. They are responsible for consuming almost all leaf litter and new seedling germinates, so that rainforest on Christmas Island looks quite interesting and unique in that it is structurally simple and open and the ground free of leaf litter for much of the year.
Unfortunately, in recent decades areas of the island have become overrun by the exotic yellow crazy ant. These ants establish a mutualistic relationship with exotic honeydew producing scale insects and form multi-queen super-colonies that overwhelm and kill all red land crabs within an area. This local extinction of a real keystone species causes significant changes to the properties of the ecosystem. Leaf litter builds up and is able to persist throughout the year, seedlings recruit en masse, which increases habitat complexity, and as the red crab is an omnivorous species, there is also the creation of enemy-free space through the removal of a potential predator.
So the main question of my project: do successful invaders (in this case the yellow crazy ant and scale insects) facilitate the invasion success of other previously unsuccessful invaders as a result of altering properties of the recipient community? To address this I looked at the exotic land snails on Christmas Island, a community that comprises of around 22 species in which the smallest are one-millimetre-long litter dwellers, and the largest are the giant African land snails which can be as big as the palm of your hand.
From sampling across the island, I found that although many of these snails were present in intact forest where crabs were common, in areas where crabs had been removed by the crazy ants, these exotic snails were orders of magnitude more abundant, the difference of only a few individuals versus hundreds. This suggests that the majority of these exotic snails was small enough to escape predation by the land crab but were limited by the lack of habitat and resources that intact rainforest on Christmas Island offered. The changes in properties of the recipient community as a result of yellow crazy ant invasion created conditions whereby populations of these exotic snails were released, and therefore able to increase in abundance, spread and become invasive.
It’s my aim that by untangling these complex species interactions I can contribute to the kinds of important data needed to better understand the impacts of successful invaders and their potential indirect role in facilitating the invasion success of others, a process called secondary invasion. As I mentioned at the start, in a time where humans are constantly moving species all over the globe, understanding novel species interactions in altered ecosystems is highly important to our conservation efforts and the maintenance of healthy ecosystems.
Robyn Williams: Luke, that’s amazing, the fact that the land crabs are so important is kind of well known, but their absence causing such disruption…now, if you are at La Trobe, and Christmas Island is, what, the other side of the continent, better known for asylum seekers, how often do you manage to get there?
Luke O’Loughlin: I’ve actually spent the best part of the last three years there. I’ve been mostly based on Christmas Island. It’s a great spot.
Robyn Williams: It’s an amazing island, isn’t it, people forget that it’s part of Australia.
Luke O’Loughlin: Yes, it’s quite unique, it’s like a place unto itself.
Robyn Williams: Far closer to Indonesia in some respects.
Luke O’Loughlin: Yes, only 300 kilometres south of Java.
Robyn Williams: Indeed. Thanks Luke. Luke O’Loughlin finishing his PhD at La Trobe.